The ultrasonic techniques which have application to flaw detection have received wide attention in the past decade with many advances being made in field application of robotic inspection and imaging systems. Such techniques have addressed the problem of inspecting for flaws that are located in regions where ultrasonic inspection is complicated by the fact that a wide variety of flaws must be differentiated from other benign ultrasonic indications. A number of techniques have been devised to detect, discriminate and size such defects, for example, time-of-flight diffraction, synthetic aperture focusing, artificial neural networks and the pattern classification of frequency and time domain waveform features.
The time-of-flight diffraction (TOFD) technique measures the time of reflection of the initial signal from the internal flaw or defect, as well as, the signal reflected from the backwall of the specimen being tested. (see Charlesworth, J. P. and J. A. G. Temple, Engineering Application of Ultrasonic Time-Of-Flight Diffraction, John Wiley & Sons Inc., pp 14-15, 1989).
The synthetic aperture focusing technique (SAFT) is a process in which the focal properties of a large-aperture, focused transducer are synthesized from a series of measurements made using a small-aperture transducer which has been scanned over a large area. (see Busse, L. J., H. D. Collins and S. R. Doctor, Review and Discussion oil the Development of Synthetic Aperture Focusing Technique for Ultrasonic Testing (SAFT-UT), Pacific Northwest Laboratory, NUREG/CR-3625 PNL-4957, pp 11 and 16, 1984).
Artificial neural networks (ANN) have been used to generate integrated average measures of microstructural properties of the specimen by examining the power spectral density distributions. (see Thomsen, J. J. and K. Lund, "Quality Control of Composite Materials by Neural Network Analysis of Ultrasonic Power Spectra", Materials Evaluation, May 1991, pp 594-600). Note that the RF waveform is required to extract waveform features from power spectral density distributions.
Pattern Classification relies generally on various signal-processing techniques that are well know in the art. (see Shankar, R., P. Jeong, F. Ammirato, L. Nottingham, G. Henry, S. Liu and M. Avioli, "Signal-Processing Applications in the Electric Utility Industry", Materials Evaluation, November 1990, pp 1365-1373 and 1391). Also note that two companies, Tektrend (Montreal, Canada) and Infometrics (Silver Spring, Md.) both have commercial products sold under the trademark names of, ICEPAK and TESTPRO, respectively, that implement feature analysis and pattern classification for ultrasonic applications.
A generic requirement of each of these techniques is that the ultrasonic RF signal is digitized at precise position grid points in order to preserve sufficient signal and spatial information to permit subsequent analysis (and reanalysis) using any appropriate signal processing algorithm.
At present, portable ultrasonic inspection is carried out with the use of portable flaw detectors which are operated in a manner similar to standard oscilloscopes and probes, namely, the flaw detector is set down near the test piece and a transducer and cable are connected to it. This poses both a control problem and a display problem.
The control problem arises because the operator typically must use one hand to "scrub" the test piece with the transducer and the other hand for support. This means that the one free hand (non-supporting) is available to either scan the test piece or operate the instrument. The display problem arises whenever the operator cannot place the instrument and the test piece in the same field of view; then, the operator can look at either the transducer or the display, but not both simultaneously. This presents an operational problem in that the operator must keep the transducer stationary, but out of sight, while making instrument adjustments and observing the displayed consequences. Only in ideal situations, such as on a lab bench, can both the transducer and the test instrument be operated simultaneously with two hands in full view of each other.
At present, only simple flaw detectors are sufficiently portable for the applications envisioned. Yet, apart from the operational logistic problems in their use, they offer very limited data recording capability; either RF data is not recorded at all, or if they are, these data are not position referenced nor are they recorded in sufficient quantity to produce useful images.
Computer based ultrasonic imaging systems are typically transportable rather than portable. The smallest such systems weigh about 50 pounds and are not battery powered. One portable system, the Acoustic Crack Detector (ACD), developed for the Department of Transportation about five years ago, represents the current state of the art. However, it suffers from the fact that two operators are required. One is the "up-hole" operator stationed in a mini-van with all of the ultrasonic, computer and electrical equipment. The "up-hole" operator is connected via a long umbilical cable to the "down-hole" operator. The "down-hole" operator scrubs the test piece with the transducer and can only observe a transmitted image on the instrument display, but can not directly control the ultrasonic instrument itself. To effect instrument changes, he must request changes using a voice telecommunication channel with the up-hole operator.
The various instruments that are currently on the market, as illustrated above, have several shortcomings that limit the use of these instruments in some of the most critical environments and areas of limited access. To over come these problems and limitations it would be advantageous to have an instrument that: permits the inspector to view the inspected part and the test image simultaneously; has a semi-automatic scanner so that the inspector only has to move the probe in one direction while motion in the second orthogonal direction is provided by an internal motor; permits a single person to wear the complete system allowing him access to the smallest of areas and the ability to use such a system hands free so that he has two hands to assist him in climbing in tight areas; that has the ability to acquire and store a digital photographic image of the surface being tested; and is totally self contained and does not require attachment to other remote components to complete the system. The present invention provides each of these advantages.